Glaucoma is one of the most major eye diseases globally, and causes irreversible loss of vision due to the optic nerve damage leading to blindness. It is largely caused by poor filtration of aqueous fluid in the eyeball through the anterior chamber angle (ACA). If untreated, it leads to higher internal pressure, permanent nerve damage and blindness. It is the second leading cause of global blindness after cataract and is the leading cause of irreversible visual loss [1]. It accounts for 40% of blindness in Singapore [2].
There are two main types of glaucoma, depending on how the flow of fluid is blocked:                Open-angle glaucoma is caused by a gradual hype-functioning of the trabecular meshwork.        Angle-closure glaucoma (ACG) is caused by a change in the position of the iris, which then occludes the drainage channels. This is shown in FIG. 1, where the top left part indicates by a rectangular box the portion of the eye being considered, the top part of the figure represents a normal eye, and the lower part of the figure illustrates angle closure.        
Glaucoma is asymptomatic in an early stage and is often only recognized when the disease is quite advanced and vision is lost. Detection of ACG in the early stage using clinical imaging modalities could lead to treatment to arrest its development or slow down the progression.
Anterior chamber angle assessment is used for the detection of ACG and is essential in deciding whether or not to perform laser iridotomy. Three approaches are used, namely, gonioscopy, ultrasound biomicroscopy (UBM) and anterior segment optical coherence tomography (AS-OCT).
Gonioscopy involves imaging with a contact lens placed onto the eye. Though considered as ‘gold standard’, gonioscopy is highly subjective. The definition of angle findings varies across grading schemes and there is no universal standard. It is also prone to potential measurement errors due to how the lens is placed on the eye [5] and different illumination intensities. It is also uncomfortable for the patient. As such, there are severe constraints to its potential as a screening tool.
Ultrasound biomicroscopy (UBM) uses a higher frequency transducer than regular ultrasound for more detailed assessment of the anterior ocular structures [6]. The different parameters defined in [7] to quantify ACA are as follows:                Angle opening distance (AOD): AOD 250/500 is the length of the line segment (referred as AOD line) drawn from a point which is 250/500 μm away from the scleral spur on the corneal endothelium, to the anterior surface of the iris in the normal direction of the endothelium. AOD 250 was first reported by [7] but is currently rarely used due to the high variability of the iris configuration at this location. In [8], AOD 500 is considered to be a better estimate of ACA.        Angle recess area (ARA): ARA refers to the area bounded by corneal endothelium, the iris and the AOD line at 500 or 750 μm.        Trabecular iris space area (TISA): TISA is the area between the scleral spur and the AOD line. The TISA modifies the ARA by removing the area anterior to scleral spur.        
Ishikawa et al. designed a semi-automated program (UBM Pro2000) [9] to calculate several important parameters, i.e. AOD 250, 500 and ARA 750, based on the manual identification of the scleral spur, which is prone to intra-observer and inter-observer variability. Although UBM is useful in quantifying the ACA, the equipment is costly and the resolution is sometimes unsatisfactory. Furthermore, it is neither user nor patient friendly as a water bath is needed to image the eye [5].
AS-OCT is another instrument for imaging the anterior chamber angle. Optical coherence tomography is analogous to ultrasound imaging, as the image is formed by detecting the signal backscattered from different tissue structures. Instead of sound waves, light is used for OCT imaging, which avoids the need for direct contact with the eyes to transmit or receive the signal. Furthermore, the use of light achieves higher spatial resolution than ultrasound. From the experiments in [10], AS-OCT is found to be at least as sensitive in detecting angle closure when compared with gonioscopy.
The existing angle assessment parameters used in AS-OCT remain the same as UBM images. The current Visante™ built-in angle assessment software requires substantial user labeling, i.e. the scleral spur, cornea and iris, hence the measurements are subjective. The Zhongshan Angle Assessment Program [11] is able to define the borders of the corneal epithelium, endothelium and iris to measure AOD, ARA and TISA using the location of scleral spur as the only observer input. However, it is found that the scleral spur is not identified in 20% to 30% of Visante™ OCT images and measurements using the scleral spur as the landmark are subjective to intra-observer and inter-observer variability and so are not very reproducible.
With the advancement in OCT imaging technology, higher resolution images could be produced in shorter time. Zeiss Cirrus™ HD-OCT [12] uses spectral domain technology to achieve image acquisition speed of 27,000 axial scans per second, which results in approximately 50 times faster data acquisition in practice as compared to the time-domain Visante™ OCT. Furthermore, the transverse resolution of HD-OCT images is improved from 60 μm/pixel to 15 μm/pixel and axial resolution is improved from 18 μm/pixel to 5 μm/pixel [13], [14]. However, as Cirrus™ uses a shorter wavelength (840 nm) than Visante™ (1310 nm) to achieve better spatial resolution, the penetration depth is decreased.
FIG. 2 show an anterior segment HD-OCT images. It is marked to illustrate the locations of the angle recess (the region between the cornea and the iris), the scleral spur (the point where the curvature of the angle wall changes, often appearing as an inward protrusion sclera), the corneal endothelium (the inner-most layer of cornea), the coneal epithelium (the outer-most layer of the cornea), Descemet's membrane (the second innermost layer), and Schwalbe's line (the termination of Descemet's membrane). As illustrated in FIG. 2, the angle recess is obscured in shadow and the scleral spur is not well defined in the HD-OCT image [15] due to the scattering by the sclera [14]. On the other hand, Schwalbe's line, which marks the termination of Descemet's membrane, can be identified in more than 90% of HD-OCT images [15].